Jamie Gunn heard the 82-year-old woman shouting after Shaun Morris stole her purse in Doncaster Road, Wakefield on the morning of February 23.
She had just walked from a nearby newsagents where she was pleased to discover she had won a little money on the Lottery, Philip Standfast, prosecuting, told Leeds Crown Court yesterday.
The pensioner put her purse into her pocket and, with the aid of her walking stick, made her way to a nearby pedestrian crossing.
As she waited there Morris struck up a conversation beside her then suddenly pushed her hard causing her to lose her balance and strike her shoulder on the crossing post.
At the same time he reached into her pocket and snatched the purse before running off. Mr Gunn heard the pensioner's call for the robber to stop and along with two workmen gave chase.
Mr Standfast said Morris was seen to throw the purse aside and it was later recovered but Mr Gunn continued to chase him and eventually caught up, only for Morris to threaten that he had a syringe.
Jailing Morris for 27 months, Judge Alastair McCallum said it must have crossed Mr Gunn's mind he meant with AIDS or hepatitis or something similar, but he bravely restrained the robber.
Morris, 40, of Edgemore Road, Hall Green, Wakefield, admitted robbery. The court heard he handed back 5 he had taken out of the purse so the pensioner got her property back.
Peter Byrne, for Morris, who had previous convictions for robbery, said he had a history of mental problems and felt his life was spiralling out of control.